A gauge of this type, incorporated into a valve, is described for example in EP 0,709,656. The gauge consists of an elongate rod and of a cylindrical tube coaxially surrounding said rod, the tube and the rod being borne by the body of the valve and extending over the entire height of the container. The external tube electrode is borne directly by the base, into which it is screwed. The central electrode, a solid rod, is supported by the external electrode and by gaskets to ensure electrical insulation. Struts are distributed along the rod to maintain a constant measurement space between the two electrodes. This is because the measurement principle of such a gauge is based on the variation in capacitance measured between the electrodes as a function of the level of fluid in the inter-electrode space and thus in the container.
Though such a capacitive gauge design has proven itself in practice, it does have a number of disadvantages. The electrode requires meticulous fixing, then function and leak testing, meaning that the electrodes have to be factory-fitted. The consequence is that the device is then bulky, in particular from the point of view transport thereof, in particular when the electrodes are long.